Caspase-8 mediates IMiD-induced apoptosis.

(A) Quantification of caspase-8 (black bars) and caspase-9 (white bars) activity was performed using the ApoAlert caspase colorimetric assay kit in MM.1S cells treated with IMiD1 (1 μM for 72 hours in 1% serum) after normalization for cellular protein. Treatment with Dex (1 μM for 48 hours) activated caspase-9 and served as a positive control (gray bar). Data shown (mean ± SD) are representative of 3 experiments. (B) The caspase-8–specific inhibitor IETD-FMK, but not the caspase-9 inhibitor LEHD-FMK (both used at 20 μM), protected MM.1S cells from cell death induced by IMiD1 (1 μM for 72 hours in 1% serum), as quantified by the MTT assay (black bars). LEHD-FMK attenuated apoptosis induced by Dex (1 μM for 48 hours; white bars) and served as a positive control. Data shown (mean ± SD) are representative of 3 experiments. (C) The caspase-8–specific inhibitor IETD-FMK (20 μM) protected OCI-My-5, S6B45, and primary MM patient cells from cell death induced by IMiD1 (1 μM for 72 hours in 1% serum), as quantified by the MTT assay (black bars: no inhibitor; white bars: caspase-8 inhibitor). Data shown (mean ± SD) are representative of 3 experiments. (D) The caspase-8–specific inhibitor IETD-FMK (20 μM) protected MM.1S cells from cell death induced by Thal (100 μM for 72 hours in 1% serum) or IMiD3 (1 μM for 72 hours in 1% serum), as quantified by the MTT assay (black bars: no inhibitor; white bars: caspase-8 inhibitor). Data shown (mean ± SD) are representative of 3 experiments.